3 INDUCTION MOTORS

BY:Nagaraju A.,

Asst. Prof.,EEE Dept.,

GITAM University.

Why induction motor (IM)? Advantages:

Robust High Power/Weight ratio compared to Dc motor Lower Cost/Power Easy to manufacture Almost maintenance-free, except for bearing and other mechanical parts.

Disadvantages: Essentially a “fixed-speed” machine Speed is determined by the supply frequency To vary its speed need a variable frequency supply.

ConstructionAn induction motor has two main parts:Stator

Consists of a steel frame that supports a hollow, cylindrical core.

The core is constructed from stacked laminations having a number of evenly spaced slots.

Construction cont…Slots provide the

space for the stator winding.

The conductors are connected to form a balanced 3 phase star or delta connected circuit.

Squirrel-cage rotor :

Construction cont …

Conducting bars laid into slots and Bars shorted at both

ends by end rings This forms a short

circuited winding which is indestructible Entire rotor

construction resembles an squirrel cage Simple and robust

construction Low starting torque

The rotor mounted on the shaft is of two types:

Slip ring rotor :

Construction cont …

Complete set of three-phase windings exactly as the stator

Usually Y-connected The ends of the three rotor

wires are connected to 3 slip rings on the rotor shaft.

In this way, the rotor circuit is accessible for external resistances.

External resistances are connected only during starting and removed when motor attains rated speed

Costly High starting torque

Construction cont …

Rotating Magnetic FieldBalanced three phase windings, i.e. mechanically displaced 120 degrees from each other, fed by balanced three phase source.

This field is such that its poles do not remain in a fixed position on the stator but go on shifting their positions around the stator

A rotating magnetic field with constant magnitude of 1.5 m is produced

The magnetic field rotates with a speed.

Where Fs is the supply frequency andP is the no. of poles and

Ns is called the synchronous speed

Rotating Magnetic Field Cont..

Principle of Operation When 3 stator winding is energised from a 3 supply, a

rotating magnetic field is developed . This rotating magnetic field cuts the rotor windings and

produces an induced voltage in the rotor windings which are now stationary.

Due to the fact that the rotor windings are short circuited, for both squirrel cage and wound-rotor, and induced current flows in the rotor windings

The rotor current carrying conductors are now placed in stator magnetic field

A torque is produced as a result of this action The rotor starts rotating in the same direction as the rotating

magnetic field This happens due to the LENZ law. The induced current direction is so as to oppose the cause The cause is relative speed between field and the rotor

conductors

Induction motor speed At what speed will the IM run?

Can the IM run at the synchronous speed, why?

If rotor runs at the synchronous speed, which is the same speed of the rotating magnetic field, then the rotor will appear stationary to the rotating magnetic field and the rotating magnetic field will not cut the rotor.

So, no induced current will flow in the rotor and no rotor magnetic flux will be produced so no torque is generated and the rotor speed will fall below the synchronous speed

When the speed falls, the rotating magnetic field will cut the rotor windings and a torque is produced.

Induction motor speed So, the IM will always run at a speed lower than the

synchronous speed The difference between the synchronous speed and

the motor speed is called the Slip. It is usually expressed as a percentage of synchronous speed i.e.

Where Nslip= slip speedNs= speed of the magnetic fieldNr = mechanical shaft speed of the motor

If the rotor runs at synchronous speed, then S= 0 If the rotor is stationary, then S = 1 The slip is a ratio and doesn’t have units

Rotor FrequencyThe frequency of the voltage induced in the rotor is given by:

Where Fr= the rotor frequency Fs= the stator frequency S = slip

When the rotor is blocked (s=1) , the frequency of the induced voltage is equal to the supply frequency

On the other hand, if the rotor runs at synchronous speed (s = 0), the frequency will be zero

Rotor emf, current and power at slip ‘s’

Induced voltage in the rotor = SE2,o Frequency of rotor current = SFsReactance of the rotor circuit = SX2,o Rotor current (I2) at slip (s) = Rotor power factor Cos2= Power input =

TorqueTorque derivation:

= The equation of torque for 3ph induction motor:

Starting Torque (Tst) : Starting torque depends on supply voltage and rotor resistance

Full Load Torque (Tf) : [Differentiate torque equation with respect to slip(S) and equate it to zero]

Torque Cont…

For low values of slip T sFor high values of slip T

Torque Vs Slip Characteristics

For Different Rotor Resistances

For Different Voltages

Power Flow Diagram

Power Relations Stator power i/p = stator power o/p + stator core and

copper lossStator power o/p = rotor power i/p = Air gap power

(Pa)Mechanical power = Gross rotor output power (Pg)Air gap power = gross output power + rotor copper loss

(Pr)Shaft output power (Psh)= Actual output power Gross output power = shaft power + mechanical lossesMechanical losses = Friction losses + windage losses

Efficiency () =

Induction Motors and Transformers

Both IM and transformer works on the principle of electromagnetic induction

Transformer: voltage applied to the primary windings produce an induced voltage in the secondary windings

Induction motor: voltage applied to the stator windings produce an induced voltage in the rotor windings

The difference is that, in the case of the induction motor, the secondary windings can move

Due to the rotation of the rotor (the secondary winding of the IM), the induced voltage in it does not have the same frequency of the stator (the primary) voltage.

Due to rotor rotation the losses are more compared to transformer

Need for starters Three phase induction motor is self starting machine.

The equivalent circuit of the three phase induction motor at the time of starting is like an electrical transformer with short circuited secondary winding,

Because at the time of starting, the rotor is stationary and the back emf due to the rotation is not developed

Hence the motor draws the high starting current. We use starters in order to limit this high starting current.

Types of Starters

We can employ all the methods that are used for starting of the squirrel cage induction motor to start the wound rotor motors. In addition we also have Rotor resistance starting method

Starting methods for

squirrel cage motor

Full Voltage Method

Direct on line starting method

Reduced Voltage Method

Stator Resistor Starting method

Auto transformer

starting method

Star Delta starting method

Direct On line Starting In this method we directly switch the stator of the three phase squirrel cage induction motor on to the supply mains.

The motor at the time of starting draws very high starting current (about 5 to 7 times the full load current) for the very short duration.

The amount of current drawn by the motor depends upon its design and size.

But such a high value of current does not harm the motor because of rugged construction of the squirrel cage induction motor.

Direct On line Starting Cont…..

Such a high value of current causes sudden undesirable voltage drop in the supply voltage.

A live example of this sudden drop of voltage is the dimming of the tube lights and bulbs in our homes at the instant of starting of refrigerator motor.

Generally used for small motors (less than 5 Hp)

Less costly but not good method

Stator resistance starting In this method we add resistor or a reactor in each phase as shown in the diagram (between the motor terminal and the supply mains).

Thus by adding resistor we can control the supply voltage.

Only a fraction of the voltage (x) of the supply voltage is applied at the time of starting of the induction motor.

The value of x is always less than one.

As the motor speeds up the reactor or resistor is cut out from the circuit

Stator resistance starting Finally the resistors are short circuited when the motor reaches to its operating speed.

Due to the drop in the voltage the starting torque also decreases.

If we add series resistor then the energy losses are increased so it’s better to use series reactor in place of resistor because it is more effective in reducing the voltage

however series reactor is more costly than the series resistance.

Auto transformer Starting As the name suggests in this

method we connect auto transformer in between the three phase power supply

The auto transformer is a step down transformer hence it reduces the per phase supply voltage from V1 to xV1.

The reduction in voltage reduces current from Is to xIs.

After the motor reaches to its normal operating speed, the autotransformer is disconnected and then full line voltage is applied.

Low power loss, low starting current, less radiated heat (>> 25 Hp)

Star Delta Starting This method is used for the motors

designed to operate in delta connected winding.

The stator phases are first connected to the star by the help of triple pole double throw switch (TPDT switch) in the diagram the position is marked as 1

when the steady state speed is reached the switch is thrown to position 2 ( Delta connection) as shown in the above diagram.

In the first position the terminals of the motor are short circuited and in the second position from the diagram the terminal a, b and c are respectively connected to B, C and A.

The starting torque is reduced by 66 .7 % and the reduction in voltage is fixed (5-25 Hp)

Starting Methods Of Wound Rotor Motors We can employ all the methods that

we have discussed for starting of the squirrel cage induction motor in order to start the wound rotor motors.

Rotor Resistance Starting Method: At the time of starting of the motor,

the entire external resistance is added in the rotor circuit.

Then the external rotor resistance is decreased in steps as the rotor speeds up.

Under normal condition when the motor develops load torque the external resistance is removed

This will decrease the starting current, increases the starting torque and also improves the power factor.

Speed ControlA 3 phase induction motor is basically a constant speed motor

So it’s somewhat difficult to control its speed.

The speed control of induction motor is done at the cost of decrease in efficiency and low electrical power factor.

Speed Control Types

Speed Contro

l

Stator Side

V / F Method Supply Voltage MethodStator poles

MethodStator

resistance Method

Rotor Side

Rotor resistance

MethodCascade Method

Slip frequency emf into

rotor Method

V / F control or frequency control

In three phase induction motor emf is induced by induction similar to that of transformer which is given by

Now if we change frequency synchronous speed changes but with decrease in frequency flux will increase and this change in value of flux causes saturation of rotor and stator cores which will further cause increase in no load current of the motor .

So its important to maintain flux constant and it is only possible if we change voltage

If we decrease frequency but at the same time if we decrease voltage causes flux to remain constant.

We are keeping the ratio of V/ f as constant. Hence its name is V/ f method.

For controlling the speed of three phase induction motor by V/ f method we have to supply variable voltage and frequency which is easily obtained by using converter and inverter set.

Supply Voltage MethodThe torque produced by running three phase induction motor at low slip is given by:

From the equation above it is clear that if we decrease supply voltage torque will also decrease.

But for supplying the same load, the torque must remains the same and it is only possible if we increase the slip and if the slip increases the motor will run at reduced speed .

This method of speed control is rarely used because small change in speed requires large reduction in voltage, and hence the current drawn by motor increases, which cause over heating of induction motor.

Stator Poles MethodMultiple stator winding method:In this method of speed control of three phase induction motor , the stator is provided by two separate winding.

These two stator windings are electrically isolated from each other and are wound for two different pole numbers.

Using switching arrangement, at a time , supply is given to one winding only and hence speed control is possible.

Disadvantages of this method is that the smooth speed control is not possible . This method is more costly and less efficient as two different stator winding are required.

Stator resistance MethodIn this method of speed control of three phase induction motor rheostat is added in the stator circuit due to this voltage gets dropped .

In case of three phase induction motor torque produced is given by T ∝ sV2

2. If we decrease supply voltage torque will also decrease.

But for supplying the same load , the torque must remains the same and it is only possible if we increase the slip and if the slip increase motor will run reduced speed.

Rotor Resistance Method In this method of speed control of three phase induction

motor external resistance are added on rotor side. Now if we increase rotor resistance torque decreases but

to supply the same load torque must remains constant. So, we increase slip, which will further results in decrease

in rotor speed. Thus by adding additional resistance in rotor circuit we can

decrease the speed of three phase induction motor. The main advantage of this method is that with addition of

external resistance starting torque increases The speed above the normal value is not possible. Large speed change requires large value of resistance and

if such large value of resistance is added in the circuit it will cause large copper loss and hence reduction in efficiency.

This method cannot be used for squirrel cage induction motor.

Injecting Slip Frequency Emf Into Rotor Side

When the speed control of three phase induction motor is done by adding resistance in rotor circuit, some part of power called, the slip power is lost as I2R losses.

Therefore the efficiency of three phase induction motor is reduced by this method of speed control.

This slip power loss can be recovered and supplied back in order to improve the overall efficiency of three phase induction motor and this scheme of recovering the power is called slip power recovery scheme

This is done by connecting an external source of emf of slip frequency to the rotor circuit.

The injected emf can either oppose the rotor induced emf or aids the rotor induced emf.

If it oppose the rotor induced emf, the total rotor resistance increases and hence speed decreases

If the injected emf aids the main rotor emf the total resistance decreases and hence speed increases.

Therefore by injecting induced emf in rotor circuit the speed can be easily controlled.

The main advantage of this type of speed control of three phase induction motor is that wide range of speed control is possible whether its above normal or below normal speed.